I have had some intensive GPS experience as I recently (March 09) published a guide book that used GPS coords for adventure motorcycle trips in the Idaho/Nevada/Oregon desert (ask me for info in you are interested; unabashed plug here!)
I learned a couple of things as I rode all over the desert. The battery issue was a big one. I started with a simple etrex just for tracking, and looking back at where I'd been. I moved on to a Triton which is used in conjucntion with Nat Geo Topo. Its a great unit, small, with color screen and viewable topo maps. HOWEVER, any of the color screens EAT up batteries! I've gone to rechargable 2900 lithium batteries, and I can get about 10 hours depending on how much "looking" I do at the screen, beacause every time you check the screen it lights up, using more power. I've rarely used it for true backpacking, as I think the maps are actually more flexible, ie larger view area, more detail than a 2" square. I can see using a small non color GPS just for waypoint location, so you can pre-plot your off trail turning points, and then using map and compass, decide on the route once your really there. But as for having your space age GPS in hand, well, not quite to Star Trek level yet! BTW, Nat Geo Topo is superb, as you can make your own maps. I swear by adventure paper! I make maps that encompass just the areas I need, and its a great way to go.
I haven't found a way to really use one for a multi day trip that won't run you heavy with battery weight.